1. Colonies of the coli-typhoid group are natural diffraction gratings. 2. The slits of such gratings are circular. 3. Such gratings produce circular spectra. 4. The distances between the slits of the grating's are equal to the transverse diameter of the respective bacilli. 5. The transverse diameter of a bacillus can be calculated from its diffraction spectrum. 6. In a colony of the coli-typhoid group all the bacilli stand upright, at right ang-les to the surface of the medium.